Ultrasound contrast agents are used routinely in medical diagnostic, as well as industrial, ultrasound. For medical diagnostic purposes, contrast agents are usually gas bubbles, which derive their contrast properties from the large acoustic impedance mismatch between blood and the gas contained therein. Important parameters for the contrast agent include particle size, imaging frequency, density, compressibility, particle behavior (surface tension, internal pressure, bubble-like qualities), and biodistribution and tolerance.
Gas-filled particles are by far the best reflectors. Various bubble-based suspensions with diameters in the 1 to 15 micron range have been developed for use as ultrasound contrast agents. Bubbles of these dimensions have resonance frequencies in the diagnostic ultrasonic range, thus improving their backscatter enhancement capabilities. Sonication has been found to be a reliable and reproducible technique for preparing standardized echo contrast agent solutions containing uniformly small microbubbles. Bubbles generated with this technique typically range in size from 1 to 15 microns in diameter with a mean bubble diameter of 6 microns (Keller et al. 1986. J. Ultrasound Med. 5:493-498). However, the durability of these bubbles in the blood stream has been found to be limited and research continues into new methods for production of microbubbles. Research has also focused on production of hollow microparticles for use as contrast agents wherein the microparticle can be filled with gas and used in ultrasound imaging. These hollow microparticles, however, also have uses as drug delivery agents when associated with drug products. These hollow microparticles can also be associated with an agent which targets selected cells and/or tissues to produce targeted contrast agents and/or targeted drug delivery agents.
Surfactant stabilized microbubble mixtures for use as ultrasound contrast agents are disclosed in U.S. Pat. No. 5,352,436.
WO 9847540 discloses a contrast agent for diagnostic ultrasound and targeted disease imaging and drug delivery comprising a dispersion of a biocompatible azeotropic mixture, which contains a halocarbon.
WO 9421301 discloses an ultrasound agent consisting of a biocompatible oil-in-water emulsion in which the oil phase comprises an oil-soluble gas/fluid or gas precursor.
U.S. Pat. No. 5,637,289, U.S. Pat. No. 5,648,062, U.S. Pat. No. 5,827,502 and U.S. Pat. No. 5,614,169 disclose contrast agents comprising water-soluble, microbubble generating carbohydrate microparticles, admixed with at least 20% of a non-surface active, less water-soluble material, a surfactant or an amphiphillic organic acid. The agent is prepared by dry mixing, or by mixing solutions of components followed by evaporation and micronizing.
U.S. Pat. No. 5,648,095 discloses hollow microcapsules for use in imaging and drug delivery. The hollow microcapsules are made by combining a volatile oil with an aqueous phase including a water soluble material such as starch or a polyethylene glycol conjugate to form a primary emulsion. The primary emulsion then is combined with a second oil to form a secondary emulsion, which is hardened and allows for microcapsules to form around a liquid core of the volatile oil. The volatile oil is then removed by evaporation leaving a hollow microcapsule.
U.S. Pat. No. 5,955,143 discloses hollow polymer microcapsules that are produced by dissolving a film-forming polymer in a volatile non-aqueous solvent, dispersing into the polymer solution finely divided particles of a volatilizable solid core material, inducing formation of a solid polymer coating on the particulate solid core material to produce polymer microcapsules having an encapsulated solid core. This core is then removed to result in hollow microcapsules that can be then filled with gas for contrast imaging.
There remains a need for methods of production of biocompatible, biodegradable echogenic microcapsules and nanocapsules of a reproducible size range that can be used for contrast imaging and/or drug delivery with or without targeting capabilities.